Heretofore, in an electrostatic recording process with copying machines or printers, first, a surface of a photosensitive body (latent image carrier) is uniformly electrified. An image is then projected onto the photosensitive body by an optical unit to remove electric charges corresponding to the image, thus forming an electrostatic latent image. Toner fed to the surface is electrostatically deposited on the electrostatic latent image to form a toner image. The toner image is then transferred onto a recording medium, such as paper, a transparency for OHR, or photographic paper, for printing.
The same procedure is basically followed in printing with color printers or color copying machines. Furthermore, in color printing, four types of color toner of magenta, yellow, cyan, and black are used to reproduce colors. Thus, these four types of toner must be superimposed at a predetermined ratio to reproduce colors. Several methods have been proposed for this color reproduction process.
One of the methods is image-on-image development. In this method, when an electrostatic latent image is visualized by feeding toner onto a photosensitive body, as in monochrome printing, the four types of color toner of magenta, yellow, cyan, and black are sequentially superimposed to develop the electrostatic latent image, thus forming a color toner image on the photosensitive body. Although this method can reduce the size of apparatus, it is very difficult to control the gradation by the method. Thus, high image quality is rarely achieved.
A second method is a tandem method including four photosensitive drums. In this system, each latent image on the four photosensitive drums is developed with magenta, yellow, cyan, or black toner, thus forming a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image. The four toner images formed on the respective tandem photosensitive drums are sequentially transferred onto a recording medium, such as paper, to produce a superimposed color image. This second method can provide a high-quality image. However, the tandem method requires a series of four photosensitive drums, and a charging mechanism and a developing mechanism for each of the photosensitive drums. This increases the size and the cost of the apparatus.
FIG. 2 illustrates an example of a print section in a tandem image-forming apparatus. Four print units for yellow Y toner, magenta M toner, cyan C toner, and black B toner are disposed in series. Each of the print units is composed of a photosensitive drum 1, a charging roller 2, a developing roller 3, a developing blade 4, a toner-supply roller 5, and a cleaning blade 6. A sheet is conveyed on a transferring and conveying belt 10, which is driven by a drive roller (drive unit) 9. The four types of toner are sequentially transferred to the sheet to form a color image. The transferring and conveying belt is electrified by a charging roller 7. Electric charges on the transferring and conveying belt are removed by a charge-removing roller 8. Furthermore, a sheet is electrified by an adsorption roller (not shown) to be adsorbed on the belt. This structure can prevent the generation of ozone. A sheet is sent from a conveying path to the transferring and conveying belt by the adsorption roller, and is electrostatically adsorbed on the transferring and conveying belt also by the action of the adsorption roller. After a transferring process, a sheet is detached from the transferring and conveying belt by self-stripping utilizing the curvature of a drum, which is effected by lowering the transfer voltage to reduce the attraction force between the sheet and the transferring and conveying belt.
The transferring and conveying belt 10 may be formed of a resistive material or a dielectric material. The resistive material and the dielectric material have both advantages and disadvantages. A resistive belt retains electric charges only for a short time. When the resistive belt is used for tandem transfer, therefore, a lesser number of electric charges are injected into the belt during a transferring process. Thus, the voltage of the belt is not largely increased after the continuous transcription of four colors. Furthermore, electric charges are released before the transcription to the next sheet. It is therefore unnecessary to reset the belt electrically. However, environmental variations may change the resistance of the resistive belt, thus affecting the transfer efficiency. The thickness or the width of a sheet may also affect the transfer efficiency.
In contrast, a dielectric belt does not spontaneously release electric charges. Thus, the injection and the emission of electric charges must be controlled electrically. However, electric charges are stably held on the belt. This ensures reliable adsorption and precise conveyance of a sheet. Furthermore, the dielectric constant does not significantly depend on the temperature and the humidity. Thus, the dielectric belt allows relatively stable transferring, independent of environmental variations. One disadvantage is the accumulation of electric charges on the belt during repeated transfer. The accumulation of electric charges increases the transfer voltage.
A third method is a transfer drum method. A recording medium, such as paper, placed around a transfer drum makes four rotations, during which magenta, yellow, cyan, and black toner on a photosensitive body are sequentially transferred every rotation to the recording medium to reproduce a color image. This method can achieve relatively high image quality. However, a thick recording medium such as a thick paper sheet like a postcard is difficult to place around the transfer drum. Thus, the types of recording medium that can be used are limited.
As compared with the image-on-image development, the tandem method, and the transfer drum method, a presently proposed intermediate transfer method can achieve high image quality, does not cause an increase in the size of the apparatus, and is applicable to any recording medium.
In the intermediate transfer method, four photosensitive bodies for a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image are disposed around intermediate transfer members, including a drum or a belt, onto which the toner images formed on the photosensitive bodies are temporarily transferred. The four color-toner images are sequentially transferred to the intermediate transfer members to form a color image. The color image is then transferred onto a recording medium, such as paper. Thus, four color-toner images superimposed to control the gradation can achieve high image quality. Furthermore, unlike the tandem method, photosensitive bodies are not disposed in series. This prevents the apparatus from increasing in size. In addition, because a recording medium is not placed around a drum, any recording medium can be used.
As an example of an apparatus for forming a color image by the intermediate transfer method, FIG. 3 illustrates an image-forming apparatus including an endless belt as an intermediate transfer member.
In FIG. 3, reference numeral 11 denotes a cylindrical photosensitive body, which rotates in the direction of the arrow. The photosensitive body 11 is electrified by a primary charger 12. The projection of a light image 13 onto the photosensitive body 11 then removes electric charges on the exposed portion of the photosensitive body 11, thus forming an electrostatic latent image corresponding to a first color component. The electrostatic latent image is developed with first color toner, magenta toner M, by an image-developing unit 41, thus forming a magenta toner image on the photosensitive body 11. The magenta toner image on the photosensitive body 11 is transferred to an intermediate transfer member 20 while the intermediate transfer member 20 driven by a drive roller (drive unit) 30 is in contact with the photosensitive body 11. More specifically, the magenta toner image is transferred from the photosensitive body 11 to the intermediate transfer member 20 at a nip between the photosensitive body 11 and the intermediate transfer member 20 by the application of a primary transfer bias impressed to the intermediate transfer member 20 by a power supply 61. After the first color, magenta, toner image is transferred onto the intermediate transfer member 20, the surface of the photosensitive body 11 is cleaned with a cleaning apparatus 14, thus completing a first rotation of the photosensitive body 11 for developing and transferring. During additional three rotations of the photosensitive body, a second color, cyan, toner image, a third color, yellow, toner image, and a fourth color, black, toner image are sequentially formed every rotation on the photosensitive body 11 by developing units 42 to 44. The toner images are sequentially transferred every rotation to the intermediate transfer member 20 such that one toner image is superimposed on another, thus finally forming a composite color toner image corresponding to a desired color image on the intermediate transfer member 20. In the apparatus illustrated in FIG. 3, the developing units 41 to 44 sequentially come to the transferring position in synchronization with the rotation of the photosensitive body 11. Thus, the magenta toner M, the cyan toner C, the yellow toner Y, and the black toner B are sequentially developed.
The intermediate transfer member 20 on which the composite color toner image is formed is then brought into contact with a transfer roller 25. A recording medium 26, such as paper, is fed from a paper cassette 19 to a nip between the intermediate transfer member 20 and the transfer roller 25. At the same time, a secondary transfer bias is applied to the transfer roller 25 by a power supply 29 to transfer the composite color toner image from the intermediate transfer member 20 to the recording medium 26. The composite color toner image is thermally fixed on the recording medium 26, thus forming a final image. After the composite color toner image is transferred onto the recording medium 26, the residual toner on the intermediate transfer member 20 is removed by a cleaning apparatus 35. Thus, the intermediate transfer member 20 is reset to the initial state, preparing for the next image formation.
A fourth method is a tandem intermediate transfer method, which is a combination of the tandem method and the intermediate transfer method. FIG. 4 illustrates an image-forming apparatus for use in the tandem intermediate transfer method. A color image is formed with the image-forming apparatus having a tandem intermediate transfer member in the form of an endless belt.
The image-forming apparatus includes a first developing part 54a to a fourth developing part 54d disposed along a tandem intermediate transfer member 50. The first developing part 54a to the fourth developing part 54d develop electrostatic latent images formed on the respective photosensitive drums 52a to 52d with yellow, magenta, cyan, and black. While the tandem intermediate transfer member 50 moves in the direction of the arrow, four color-toner images formed on the respective photosensitive drums 52a to 52d in the developing parts 54a to 54d are sequentially transferred onto the tandem intermediate transfer member 50, thus forming a color toner image. The color toner image is then transferred onto a recording medium 53, such as paper, to print out.
In FIG. 4, reference numeral 55 denotes a drive roller or a tension roller for moving the tandem intermediate transfer member 50, reference numeral 56 denotes a recording medium feed roller, reference numeral 57 denotes a recording medium feeder, and reference numeral 58 denotes a fixing apparatus for fixing an image formed on a recording medium, for example, by heating. Furthermore, reference numeral 59 denotes a power supply (voltage-application means) for applying a voltage to the tandem intermediate transfer member 50. The power supply 59 can reverse the polarity of the bias voltage to transfer a toner image from the photosensitive drums 52a to 52d to the tandem intermediate transfer member 50 and a toner image from the tandem intermediate transfer member 50 to the recording medium 53.
In the image-forming apparatuses described above, electroconductive endless belts used as the transferring and conveying belt 10, the intermediate transfer member 20, and the tandem intermediate transfer member 50 are heretofore mainly semiconductive resin film belts or fiber-reinforced rubber belts. The resin film belts are, for example, electroconductive endless belts that include, as a substrate, a thermoplastic polyalkylene naphthalate resin, or a polymer alloy or a polymer blend of the thermoplastic polyalkylene naphthalate resin and another thermoplastic resin, as described in Patent Document 1.
Patent Document 2 describes a technique in which a crystalline resin having at least one selected from the group consisting of a hydroxyl group, carboxylic acid, and an ester bond, an amorphous resin having at least one selected from the group consisting of a hydroxyl group, carboxylic acid, and an ester bond, and a polymerization catalyst are heated, mixed, and then formed into a seamless belt, for the purpose of manufacturing a flexible seamless belt that exhibits no appreciable deterioration in physical properties resulting from a reaction during melt blending.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-132053 (claims etc.)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-247677 (claims etc.)